Injection laser diodes are employed extensively in optical transmission systems as the emission wavelength range of device fabricated in certain III-V alloy systems falls within the low dispersion/absorption window of single mode optical fibers. Such devices are, however, subject to output power variations as a function of operating temperature and aging conditions. Accordingly, it is important that the input power to laser diodes be controlled so as to accommodate these potential variations.
It is known that the output power of an injection laser diode as a function of injection current has two distinct regions. Below threshold the output increases relatively slowly with increasing current whereas above threshold the output power increases rapidly and substantially linearly with current. The slope of the light-current characteristic above threshold is a measure of the device efficiency. Both threshold and device efficiency are subject to variation with temperature and aging. Hence, it is important in systems employing diode lasers to have controllers which can compensate for such temperature and aging effects.